There is an urgent need for intelligent, low-energy alternatives to air-conditioning systems. Recent years have witnessed a steady increase in the amount of energy used to cool offices, commercial premises and housing. "We already use around 15 percent of our primary energy in Germany to generate energy for cooling," reports Prof. Dr. Volker Wittwer, former deputy director of the Fraunhofer ISE. And the trend is upwards: while the amount of energy required each year for cooling in Europe stood at approximately 40 terawatts in 1995, this figure is expected to triple by 2010, rising to more than 120 TWh per annum.
The ice cube effect
To produce a passive cooling effect, the researchers made use of phase change materials – known as PCMs – such as paraffin. During their transition from solid to liquid, PCMs absorb large quantities of energy, thereby preventing rooms from getting hotter. "It functions in a similar way to an ice cube: while the ice cube is melting the temperature remains at 0°C, and it doesn't rise above 0°C until everything has melted," Wittwer explains, outlining the basic principle. Paraffins melt in the comfortable room temperature range that lies between 20°C and 26°C, in the course of which they absorb massive amounts of heat from their environment and prevent the temperature from increasing. At night, when the ambient temperature drops, the wax solidifies and the capsules release the heat they absorbed, making them ready to repeat the process the next day.
The right packaging
The principle is not a new one – in fact the idea of using phase change materials to control the temperature in buildings first emerged around 60 years ago. However, attempts to incorporate PCMs in construction materials were unsuccessful for many years. The breakthrough was finally achieved when Professor Wittwer came up with the idea of packing the wax into tiny casings and integrating it in conventional construction materials such as plaster, putty and lightweight panels
Collaboration between the research community and industry
Researchers from BASF took on the task of developing the right kind of encapsulation. "We were looking for ways of encapsulating the phase change materials in microscopic containers, or ‘microcapsules’," explains Dr. rer. nat. Ekkehard Jahns from BASF. Microencapsulation offers a number of advantages: for example, the fact that the solid to liquid phase transition occurs in tiny spheres means that no wax can leak out, while the large surface areas and small volumes of the capsules means that the heat can quickly be absorbed into the material and the cold rapidly released. The diameter of the microcapsules is only around 5 µm, which is less than half the thickness of a human hair. "That makes it easier for us to incorporate the spheres in construction materials such as gypsum plaster, which can be applied to the wall in whatever form is required. The plaster does not look any different from conventional materials," Jahns continues. "And there are plenty of other construction materials that are suitable for the integration of microcapsules, such as aerated cement blocks, plasterboard and wood products."
Range of applications
The new construction materials are of particular interest for lightweight structures. A layer of PCM plaster approximately 1.5 cm thick has the same heat capacity as a concrete or brick wall. "That means we can reap the benefits of lightweight design while still storing heat," states Dr.-Ing. Peter Schossig from ISE. "Modern phase change materials help us to go a long way towards solving the problem of rooms overheating, not only in offices but also in portable prefabricated buildings and older-style loft apartments. Newly developed construction materials containing microencapsulated latent heat storage materials can make a major contribution towards enhancing buildings, especially when it comes to increasing thermal comfort and making spaces more comfortable," Schossig emphasizes.
Suitability for practical applications
Construction materials containing microencapsulated latent heat storage materials have already proven their suitability for practical applications. They have been incorporated in numerous buildings, including the Badenova building in Offenburg and the Haus der Gegenwart (Contemporary House) in Munich. Although the raw materials are available for purchase under the brand name Micronal PCMR, they are not yet available to buy in home improvement centers. "The explanations they require are still too elaborate at this point. The key is to integrate the new construction materials in the building's energy concept right from the planning stage," Schossig stresses. But how long will these new building materials last? "The materials have a lifespan of between 30 and 50 years," Schossig states. They also offer further advantages, such as the fact that they do not require maintenance and do not suffer damage as a result of hammering in nails or drilling holes.
Major benefits from tiny spheres
"PCMs offer enormous economic potential. By 2050 we are hoping to cut energy consumption by 50 percent, and much of these energy savings will have to come from buildings. To do this efficiently, we need new technologies, and our materials will make a major contribution towards developing them," declares Professor Volker Wittwer with conviction.